[Competition intensity of Mongolian pine forests in Hulun Buir sand region of inner Mongolia, China].

By using totally locating method and single tree competition index model, this paper studied the competition intensity of Mongolian pine forests in Hulun Buir sand region of Inner Mongolia, with the fire-disturbed Mongolian pine forest as a case. The results showed that within the same sampling plots, dead trees had the similar competition intensity, while the accompanying species in living and whole stems had two or three times higher competition intensity than Mongolian pine. The competition stress of Mongolian pine mainly came from intra-specific, and that of accompanying species mainly came from inter-specific. The competition intensity of living Mongolian pine was significantly lower in fire-disturbed than in undisturbed plot, and the competition intensity (CI) of living stems had a power functional relationship with their diameter at breast height (D), i.e., CI = AD(-B). It was concluded that surface fire could significantly decrease the competition intensity among the stems, which would benefit the growth and development of living stems and the culture of the stems with larger diameter of breast height.

[Distribution pattern of natural vegetation along moisture gradient at north edge of Ordos plateau: a preliminary study].

With Duguitala Township in the north part of Hangjin County as an example, this paper studied the spatial distribution pattern of natural vegetation along moisture gradient at the north edge of Ordos Plateau. The results showed that based on TWINSPAN classification, the communities were dominated by mesophytes and xerophytes, and there were some halophytes in some quadrats. The similarity indices of neighboring quadrats could reflect the mosaic pattern of different type communities, and thus, the diversity and complexity of communities along this gradient, which could be reconfirmed by DCA ordination. The CCA ordination for quadrats and geomorphological factors indicated that in this transitional region, the formation of vegetation's spatial distribution pattern was determined by moisture gradient, while the latter was controlled not by the distance from the main course of the Yellow River but by the change of micro-geomorphology. In the land exploitation of this region, relatively stable geo-morphological units should be taken into consideration, and large scale land exploitation through surface leveling should be avoided if shelterbelt measures and drainage and desalinizeation system were defective. This study would be helpful to the protection of natural vegetation and the integrated development of agriculture and animal husbandry.

[Preliminary research on environmental characteristics of oasis-desert ecotone].

Taking Minqin Oasis in the downstream of Shiyanghe Rivers as example, the shrub characteristics of sand dune, which was closely related to the vegetation of the interface between the outside of oasis and desert, was studied in this paper. The change of vegetation coverage and dune density indicated that the vegetation coverage in the ecotone was obviously higher than that in the outside of oasis and in drift desert. There existed an area outside oasis that was obviously affected by human activies and 0-600 m away from the outside of oasis. Meanwhile, the appearing position of area outside oasis was different in different place. The naked area ratio of dune in the area outside oasis was the biggest, so that the area outside oasis was the main resource of drift sand resulting in desertification of oasis. Over extracting ground water in oasis district resulted in a forming of dropping funnel of ground water there, and resulted in a fluid of ground water from desert to oasis. Ultimately, human activites aggravated those processes to some extent.